1 6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



tinually more highly illuminated than the posterior, and 

 that this difference of light intensity might determine the 

 direction of movement; he merely states that this experi- 

 ment shows that the reactions are due to ray direction 

 without defining precisely what he means. Sachs, how- 

 ever, in referring to these experiments says (1887, p. 696), 

 " Even in the case of the influence of light on the move- 

 ment of swarm spores, the important point can only be as 

 to the direction of the rays of light, not as to whether the 

 given swarm spore is illuminated more strongly in front 

 or behind." 



The excellent observations of Engelmann (1882-1883) on 

 the reactions of unicellular forms to light have a direct and 

 important bearing on the question of aggregation. Stras- 

 burger (1878) had observed that a sudden reduction of light 

 causes a definite reaction in swarm spores — " zitternde 

 Bewegung " — and others had seen similar responses to 

 sudden changes in the intensity of other stimulating agents. 

 But Engelmann seems to have been the first to point out 

 clearly the relation between such responses and aggre- 

 gation. He made detailed observations on the movements 

 of Paramecium bursaria, Euglena viridis. Bacterium photo- 

 metricum and other similar unicellular forms, in a field on 

 a slide containing a spot more highly illuminated than the 

 surrounding region. The illuminated spot, he says, acts 

 like a trap; the organisms in their random movements swim 

 into it without response, but when they reach the boundary 

 on the way out, they stop suddenly, turn back, and thus 

 remain in the illuminated area, which soon becomes crowded 

 with them. These observations are of such vital impor- 

 tance that it seems wise to emphasize them by quoting 

 directly from the author. Regarding Paramecium bur- 

 saria Engelmann says (1882, p. 393), " Ueberschreiten sie 

 z.B. zufallig die Granze von Licht und Dunkel, oder tauchen 

 sie auch nur mit der vorderen Halfte ihres Leibes eine 

 Strecke weit in das Dunkel ein, so kehren sie sofort um 

 nach dem Licht, wie wenn das Dunkel ihnen unangenehm 



